As a rule, polyethylene terephthalates are prepared by esterifying the dicarboxylic acids, preferably pure terephthalic acid, and/or transesterifying the corresponding dimethyl esters with from 1.05 to 5, and preferably from 1.8 to 3.6 mols of the diols, relative to 1 mol of the dicarboxylic acid component, in the presence of esterification catalysts and/or reaction catalysts respectively at between 150.degree. and 250.degree. C. (reaction step A) and subjecting the reaction products thus obtained to polycondensation in the presence of esterification catalysts at between 200 and 300.degree. C. under reduced pressure, preferably &lt; 1 mm Hg (reaction step B).
Catalysts thus play a central role in the preparation of polyesters. They not only have a considerable influence on the reaction rate of the transesterification reactions but also influence side reactions and the heat stability and the color of the polyethylene terephthalates. Virtually all the metals, in the form of very diverse compounds thereof, have already been used as transesterfication catalysts and polycondensation catalysts (R. E. Wilfang in Polym. Sci. 54, 385 (1961)).
Among the many known polycondensation catalysts for reaction step B, compounds of germanium, antimony and titanium are preferably used, separately or in combination. For example, U.S. Patent Specification No. 2,578,660 describes the use of germanium and germanium dioxide. Germanium compounds do indeed give polyesters with an excellent degree of whiteness but they have only an average catalytic activity.
The use of antimony compounds (in combination with phosphorus compounds as stabilizers) is known, for example from U.S. Patent Specification No. 3,441,540 and from East German Patent Specification Nos. 30,903 and 45,278.
Titanium compounds, inter alia titanium tetraisopropylate or titanium tetrabutylate, are described, as catalysts for the preparation of fiber-forming polyesters, in, for example, British Patent Specification Nos. 775,316, 777,216, 793,222 and 852,061, U.S. Patent Specification Nos. 2,727,881, 2,822,348 and 3,075,952 and (in combination with phosphorus-containing stabilizers) in East German Patent Specification No. 45,278.
Soluble antimony compounds which possess a good catalytic activity for the polycondensation reaction have the disadvantage that, under the reaction conditions, they are relatively easily reduced to metallic antimony and as a result give rise to a greyish-tinged discoloration of the polycondensate to a greater or lesser extent. According to investigations carried out by H. Zimmerman (Faserforschung and Textiltechnik 13, No. 11 (1962), 481-90), soluble titanium compounds are clearly superior to comparable antimony compounds in respect of their catalytic activity. However, they have the disadvantage that they produce a yellow-brown discoloration of the polycondensates, in particular ticular polyethylene terphthalates, if the reaction time is not kept exceptionally short (which in the case of discontinuous processes meets with considerable difficulties for reasons of apparatus) or if the catalyst concentration is not set so low that the desired degree of polycondensation cannot be achieved in times which can be tolerated industrially.
After the end of reaction step A, stabilizers are generally added to the reaction mixture and these are intended to serve to inhibit the catalysts necessary for reaction step A and to increase the stability of the end product. Such inhibitors have been described by H. Ludewig, Polyesterfasern (Polyester fibers), 2nd edition, Akademie-Verlag, Berlin 1974, in U.S. Patent Specification No. 3,028,366 and in German Offenlegungsschriften (German Published Specifications) 1,644,977 and 1,544,986. Examples of such inhibiting compounds which may be mentioned are phosphoric acid and phosphorous acid and their esters, such as trinonylphenyl phosphate or triphenyl phosphate or triphenyl phosphite. Hitherto, it has not been possible to obtain high-molecular weight, light colored polyethylene terephthalates with a viscosity of &gt; 0.7 dl/g even with the use of these stabilizers.